The invention herein relates to thermal insulation for steel mill reheating furnace skid pipes.
It is common practice in steel mills to reheat slabs and billets for hot working in different types of reheating furnaces. One type in common use is the "pusher type" reheating furnace in which slabs or billets are pushed through several heating zones within the furnace to bring them to the desired temperature (which is usually in excess of about 2000.degree. F (1100.degree. C)). The slabs or billets slide through the furnace on hollow pipes with rail like projections ("skid rails") on the top thereof. Cooling water is continuously circulated through the hollow interior of the skid pipes. Typical "pusher type" reheating furnaces and their operation are described in The Making, Shaping and Treating of Steel, (McGannon, ed.: 9th edn., 1971), especially chapters 22 and 24.
Since the furnace is commonly at a temperature of about 2200.degree. F (1200.degree. C) and the cooling water in the skid pipes is at about 60.degree. F (15.degree. C), a substantial amount of heat (on the order of about 75,000 BTU/ft.sup.2 /hr or 240,000 joules/m.sup.2 /hr) is lost through the skid pipe to the cooling water. In order to prevent this heat loss, it is desirable to thermally insulate the skid pipes. Such insulation has been readily achieved for the water cooled uprights and cross-overs which support the skid pipes in the furnaces. The skid pipes, however, have proved to present a unique problem for thermal insulation for several reasons:
1. The high temperature thermal environment of the furnace is too severe for many common insulating materials.
2. Since the slabs slide over the skid rail portion of the skid pipes, insulation cannot be wrapped entirely around the skid pipes as it can the uprights and cross overs.
3. Since the skid rails are only approximately 1 inch (2.5 cm) high, clearance required for the slabs and billets limits insulation thickness to not more than that amount. Consequently, a suitable insulation must have a high thermal efficiency per unit thickness.
4. Scale falling from the passing billets and slabs can be highly damaging to the insulation.
5. It is sometimes necessary to cool down a reheat furnace rapidly, and this is conventionally done by spraying water throughout the furnace. The thermal shock thus imparted to the insulation is destructive to the more rigid types of insulation.
6. Finally, and most importantly, a skid pipe insulation is subjected to an extreme degree of vibration. Slabs are often 6 inches (15 cm) thick, 7 feet (2.1 m) wide, and 30 feet (9 m) long; billets may be 1 foot (30 cm) square in cross-section and up to 30 feet (9 m) in length; and both may weigh on the order of 5 to 7 tons (4.5 to 6.5 metric tons). Sliding a continuous row of such massive objects along the tops of the skid rails causes severe vibration of the entire skid pipe structure, which in turn rapidly destroys the integrity of most insulations.
In the past attempts have been made to overcome these adverse factors (particularly the thermal and vibrational problems) by making skid pipe insulations of massive rigid refractory cement materials containing internal metal reinforcements or anchors. Typical is the structure shown in U.S. Pat. No. 3,848,034, where a refractory cement is anchored to studs welded to the outside of the skid pipe. The massiveness of such materials have, of course, made them very difficult to apply, for they have required substantial supports. Also, workmen have only been able to install very small segments at any one time because of the great weight to be handled. The rigidity of the finished cements have also made them highly susceptible to damage by vibration and thermal shock.
In the mid-1960's a skid pipe insulation incorporating refractory fiber was introduced by Johns-Manville Corporation under the trademark "FIBERCHROME Skid and Support Pipe Insulation". This material is a vacuum formed cylindrical sleeve comprised of a binder impregnated refractory fiber mass, reinforced for skid pipe service with an internal web of stainless steel mesh similar to that used in chainlink fencing. In a typical installation the steel mesh is welded to the skid pipe and at least a portion of the outer surface of the sleeve is covered with refractory cement.